Here is the full list of all 24 Pyramid Defense achievements.
Kill 50 mummies
Kill 500 mummies
Kill 50 convicts
Kill 500 convicts
Kill 50 InvisibleNinjas
Kill 500 InvisibleNinjas
Kill 50 Necromancers
Kill 500 Necromancers
Kill 50 Ninjas
Kill 500 Ninjas
Kill 50 Pirates
Kill 500 Pirates
Kill 50 Robots
Kill 500 Robots
Kill 50 Mercenaries
Kill 500 Mercenaries
Kill 50 Skeletons
Kill 500 Skeletons
Kill 50 Slimes
Kill 500 Slimes
Kill 50 TinySlimes
Kill 500 TinySlimes
Kill 50 Wizards
Kill 500 Wizards